In a skillet over medium heat, add the oil and onions and cook until slightly soft, about 3 minutes.
Add the ground beef and salt and pepper and cook until all the way browned. Spoon out any excess grease if necessary.
Add the lasagna noodles in an even layer on top of the ground beef mixture.
Pour the pasta sauce and the water over the pasta and give the pan a shake to make sure the liquid gets down around the noodles and the ground beef.
Reduce the heat to medium low, cover and let cook until the noodles are tender, about 20 minutes.
Uncover and add the ricotta cheese in dollops around the pan. Spread it evenly over the top.
Add the mozzarella cheese on top and cover until the cheese is melted, about 5 minutes.